There are still other plural forms out there with a fair number of representatives. Plurals formed by vowel mutation remain common: man, men; mouse, mice; foot, feet. Some plurals don't inflect, and so must be understood from context: deer; moose; fish (except when talking about different species, in which case we talk about "the fishes"). And, of course, there are those words -- mostly of Latin or Greek origin -- which form their plurals in English as they did in their original languages: stadium, stadia; alumnus, alumni; index, indices.
Then there's the -n declension. Somewhere in the confusion of Middle English, we wound up with a handful of survivals which form their plurals in -n. I can think of only five of them. First, there is ox, oxen. And there used to be an old dialectal form shoe, shoon. Those are the simple ones.
But there is also the archaic plural collective kine, meaning "cows." Today, we mostly say "cattle" when we're talking about cows collectively, but that is a borrowing from French; the French word also turned into "chattel," meaning "possession." Cows were a principal form of movable wealth in the early Middle Ages, so you can see the common origin. Old English cu meant "cow." The pronounciation of cu shifted in Middle English to become eventually our modern cow, but in the Northumbrian dialect, kye. Today, kye is both singular and plural, but somewhere in there, the plural kine was born, and we still use it, albeit rarely.
Finally, there are the relationship words children and (old-fashioned) brethren. The plural of child was, once upon a time, childer. Somewhere along the way, it acquired a final -n to become children. Sort of a double plural inflection. Why? Nobody knows. When English made this change, all the fashionable people spoke Norman French, and they didn't care how the peasants spoke, so long as they understood their orders. Meanwhile, the OE word for "brother" was broðor, and its plural was breðer. In the same way that childer became children, brether became brethren: another double plural ending.
So that's my list for -n plurals in English. Five words: oxen; shoon; kine; children; brethren. Can you think of any more?
Update: I suppose to be strictly fair, I ought to include a sixth word: sistren. It pains me to hear this word, and I would be tempted to say it's not a word at all, but the fact is, I've heard it many times. It may be a mistake, but it is used consistently by culturally Southern speakers when speaking in an elevated, religious context. And since philology aims to be descriptive rather than prescriptive and countenances no snobbery concerning dialects, I guess I'll have to address this ugly formation.
At the time the KJV was being translated, brethren was already going out of fashion. It was being replaced almost everywhere by brothers. But the KJV translators were very conservative in some ways, very conscious of the need for their Bible to sound dignified. So brethren was retained in many instances. The KJV became the basic text for literacy, especially in America, and its vocabulary and cadences deeply influenced many people, especially in what we call the Bible Belt. These folks hung onto the formation brethren (pronounced usually, bruthren or bruthern) in their discourse. And when they were speaking in their religious mode, they would/will often say sistern to match: hence, brethren and sistren in place of modern brothers and sisters.
Be it noted, sister has always taken its plural in -s. The dialectal form sistren or sistern is of very recent coinage, and as even hard-core Bible believers adopt different translations, it may not hang around much longer. We can only hope.